1. Field of the Invention
The present invention relates to offshore drilling activities. More particularly, the present invention relates to offshore drawworks that include heave compensators so as to cause the drill string to move in relation to the heave of the vessel upon which the drawworks is located. Additionally, the present invention relates to flywheels that can be used for energy storage and used, in particular, in association with the cyclic loads.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
There are many systems in current use that have a high power consumption cycle and a low power consumption cycle. These systems can include cranes, drag lines, oil well derricks, swell compensators, hopper arm gimbals and drag head controls. Quite often, in the oil production industry and in mining operations, it is quite common to require high power and energy consumption during certain portions of the operating cycle and low power consumption during another part of the operating cycle of the system. As an example, a crane used in either offshore operations or in mining operations will require very little power to the motor of the crane during the lowering of the bucket of the crane. In contrast, a great deal of power is required by the motor in order to lift a loaded bucket from a lower position to a higher position.
Conventionally, in such cyclic operations, it is necessary to size the power supply so as to accommodate the maximum expected power consumption during the high energy consumption cycle of the operation. During the cyclic loads, the power supply will continually cycle between the delivery of maximum power and the delivery of minimum power. It has been found that the cyclic loads on the power supply causes a disproportionately large amount of fuel consumption and, accordingly, adverse environmental impacts.
One particular type of cyclic operation occurs in heave-compensation drawworks. Typically, these heave-compensation drawworks will be placed upon a drilling ship. The drilling ship is utilized so as to drill for oil and gas in offshore operations. In these drilling ships, the drill string will extend through a floor of the ship and be supported by a wire rope connected to a sheave system. A winch drum is connected to an end of the wire rope so as to pay out and pay in the drill string relative to the wave action affecting the drilling ship. Since it is important to maintain proper weight-on-bit during the drilling operation, the heave-compensation system is relatively complex. For example, when there is an upward heave of the drilling ship, the winch drum should pay out the wire rope so as to maintain the drill string in a proper location below the ship and to maintain proper weight-on-bit. On the other hand, when there is a downward movement of the ship, for example, by a trough of the waves, then the winch drum will pay in the wire rope so as to prevent the drawworks from exceeding the proper weight-on-bit from the downward force caused by the downwardly moving drill ship. Typically, on these heave-compensation drawworks, the motors that are associated with the winch drum and the drawworks can only lower the drill string at a maximum predetermined rate. Preferably, the drill string should be lowered as quickly as possible. However, because of the inertia associated with each of the motors of the drawworks, the motors must be controlled so as to prevent the maximum rate of downward movement of the drill string. As such, energy consuming actions, such as the application of braking forces, are placed upon the motors associated with the winch drum even during relatively non-energy consuming activities, such as the lowering of the drill string.
When the drill string is being raised for various purposes, the motors must exert sufficient power so as to elevate the drill string at a desired rate. In certain circumstances, the drill string must be lifted so as to allow for the replacement of the bit. This requires a great deal of energy consumption since the entire weight of the drill string must be lifted. As such, the motor requirements for the drilling ship are particularly high since the motors must be sized so as to be able to lift a great deal of weight associated with the drill string. A problem is that when large motors are used for the lifting of the drill string, greater braking capacity is required since large motors will have greater inertia during the lowering of the drill string.
In the past, DC motors have been used for the paying in and out of the wire rope on such heave-compensation drawworks. These DC motors typically will require a transmission so as to carry out the proper raising and lowering activities. The DC motor is clutched out during the paying out of the wire rope. A friction-type brake is utilized so as to prevent excess speed and to prevent excess inertia of the DC motor. These friction-type brakes have included, in the past, eddy-current brakes and disk brakes. Typically, the eddy-current brake is attached to the drive of the drawworks. Whenever disk brakes are used, they will tend to wear out over time.
Recently, AC motors have been incorporated into drilling ships for the purposes of controlling the drawworks and for the operation of the winch drum. These AC motors offer the benefit of greater torque and a fixed gearbox ratio. These AC motors do not require clutches. However, they will have a restricted pay out speed. Typically, the AC motor itself is used for the braking of the motor inertia. In the past, dynamic braking resistors have been employed with the use of the AC motors so as to capture some of the braking energy.
Unfortunately, these dynamic braking resistors accumulate excess energy which needs to be burned off. In offshore facilities, this excess energy is often used for the hotel load of the facility. However, offshore operators often struggle to find extra utilities to burn off the excess energy. In the past, it has been found that this excess energy from the dynamic braking resistors can be applied to the thrusters associated with the drilling ship. In particular, the engines associated with the thrusters are powered and operated by this excess energy. When the excess energy becomes too great, then drill ship operators will often point the thrusters at each other so as to maintain a stable position in the water while burning the excess energy. Unfortunately, the use of the energy in this manner will tend to quickly burn out the engines associated with the thruster and possibly compromise the DP 2 classification of the offshore system. In offshore facilities, such as drilling ships, the loss of the DP 2 classification is critical to the offshore operator. In the event that the ship does not have the ability to properly control its position relative to the bore hole, then there can be severe repercussions associated with the loss of position. As such, all drilling ships must maintain the redundant capability of its engines and the ability to maintain position within the water.
In the past, various flywheel systems have been utilized for the control of energy loads. U.S. Pat. No. 5,712,456, issued on Jan. 27, 1998 to McCarthy et al., describes a flywheel energy storage system for operating elevators. The elevator system, having a three-phase rectifier which converts energy to a three-phase AC main to provide DC power on a bus to a three-phase inverter that drives a three-phase inductive hoist motor, utilizes the generated energy applied to a boost regulator to drive a flywheel motor generator to store the regenerated energy in the form of inertia therein. When the flywheel motor generator reaches a limiting speed, any continued regenerated energy is dumped in an energy dissipating device. During periods of high demand, the inertial energy stored in the flywheel generator is used to add energy to the DC bus to provide additional current to the three-phase inverter for driving the hoist motor. The control is provided by software embedded in a elevator computer.
U.S. Pat. No. 6,043,577, issued on Mar. 28, 2000 to Bornemann et al., describes a flywheel energy accumulator having a vertical shaft rotatably supported in a vacuum housing by superconductive magnetic axial support bearings. Lower and upper flywheels are mounted on the shaft in axially spaced relationship. A homopolar dynamic machine with a rotating magnetic field is disposed in the space between the flywheels and includes a stator supported in, or forming part of, the housing. A rotor is mounted on the shaft.
U.S. Pat. No. 6,172,435, issued Jan. 9, 2001 to J. Tanaka, teaches a flywheel power source device for converting electric energy into kinetic energy and for storing the kinetic energy by rotating a flywheel. The flywheel is supported by a rotary shaft that is rotatably mounted in a bearing in a casing. The kinetic energy is reconverted into electric energy when necessary.
U.S. Pat. No. 6,236,127, issued on May 22, 2001 to Bornemann, describes another type of flywheel energy accumulator that has a vertical shaft with the rotor of an electric motor/generator in a vacuum-type housing. Flywheels are mounted on the shaft at opposite sides of the rotor. The electric motor/generator and the flywheels are placed in modules which are mounted on top of one another.
U.S. Pat. No. 6,365,981, issued on Apr. 2, 2002 to M. Tokita, provides a power generation system with a flywheel apparatus. The flywheel apparatus has a frame, a flywheel section and an exciting section. The flywheel section has an input unit having the input shaft, first and second flywheel units having the output shaft, and first and second drive units for transmitting the rotary force of the input unit to the first and second flywheel units. The exciting section increases the flywheel effect of the flywheel section.
U.S. Pat. No. 6,819,012, issued on Nov. 16, 2004 to C. W. Gabrys, discloses a flywheel energy storage system that has an energy storage flywheel supported in a low pressure containment vessel for rotation on a bearing system. A brushless motor/generator is coupled to the flywheel for accelerating and decelerating the flywheel for storing and retrieving energy. The flywheel is rotated in normal operation at a speed such that the generator voltage is higher than the output voltage. Power supplied to the load from the generator is a regulated output that is maintained at a substantially constant voltage level by using switching regulation of the alternating current voltage generated by the generator. The switching regulation of each generator phase occurs at a frequency equal to or less than twice the frequency of the generator alternating current. As so operated, the flywheel uninterruptible power supply efficiently maintains power to an electrical load during an interruption of primary power by supplying power generated from the flywheel generator.
U.S. Pat. No. 7,078,880, issued on Jul. 18, 2006 to Potter et al., provides an energy storage flywheel voltage regulation and load sharing system. This system for regulating the voltage in an electrical distribution system includes a plurality of flywheels, motor/generators, and controllers. Each of the motor/generators is coupled to one of the energy storage flywheels and to the electrical supply system. The motor/generators each supply one or more signals representative of motor/generator operational parameters, and each motor/generator controllers receive one or more of the motor/generator operational parameter signals from each of the motor/generators. In response to the operational parameter signals, the motor/generator controllers each control the operation of one of the motor/generators in either a motor mode or a generator mode. This regulates the electrical supply system voltage and equally shares the electrical load between the motor/generators.
It is an object of the present invention to provide an energy storage system on a heave-compensation drawworks that effectively stores, absorbs and relinquishes energy.
It is a further object of the present invention to provide an energy storage system on a heave-compensation drawworks that eliminates the need for transmissions.
It is a further object of the present invention to provide an energy storage system on a heave-compensation drawworks that decreases power consumption requirements.
It is still another object of the present invention to provide an energy storage system on a heave-compensation drawworks that achieves a relatively constant load profile free of peaks and valleys from the power source.
It is another object of the present invention to provide an energy storage system on a heave-compensation drawworks which achieves the fastest drop speed possible.
It is another object of the present invention to provide an energy storage system on a heave-compensation drawworks which avoids the need for brakes.
It is still a further object of the present invention to provide an energy storage system for use on a heave-compensation drawworks that maximizes fuel savings while minimizing emissions.
It is still a further object of the present invention to provide an energy storage system on a heave-compensation drawworks that extends engine life.
It is still another object of the present invention to provide an energy storage system on a heave-compensation drawworks that avoids the use of batteries and dynamic braking resistors.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.